The objective of the proposed work is to perform feasibility studies leading to the development of a novel, new defector for scintigraphy mammography capable of improving the diagnosis of breast cancer and lowering the number of false-positive diagnoses leading to unnecessary invasive biopsies. The concept is based upon a gamma-ray detector which combines a CsI(Tl) scintillator array with a new HgI2 photodetector array to form a high energy-resolution and high spatial-resolution imaging detector. The proposed development will lead (in Phase II) to a compact device which will include a high resolution collimator, CsI(Tl) scintillator and HgI2 photodetector array coupled with the necessary front-end electronic components and mounted in a hermetic package. The development is designed to proceed through two distinct phases. In phase I we will investigate critical issues necessary to determine the technological requirements for fabricating and assembling the collimator, scintillator, and photodetector arrays, the optimal choice of components and configuration of the components, techniques for successful hybridization of the detector components, and evaluation of the performance that can be achieved with prototypical four-by-four (16-pixel) device structures. In phase II we will develop a scaled-up device with approximately 20-by-20 (400-pixels) elements on a single monocrystalline slice (1-2 in.2) of HgI2 and develop the technology for integrating all of the front-end components into a hermetic package that can be used as the basis for a small imaging device to be used for small-field-of-view scintigraphy mammography or coupled with more of the devices to form a larger image focal plane. When applied to the problem of breast imaging, the proposed CsI(Tl)/HgI2 camera with 99m-Tc-Sestamibi is a method that is capable of improving the diagnosis of breast cancer without the trauma to the patient resulting from invasive surgical procedures while at the same time saving billions of dollars in the national medical bill that are now associated with these procedures. PROPOSED COMMERCIAL APPLICATION: In the third phase we will develop commercial detector systems based upon the Phase l and Phase 2 efforts. These new detectors could lead to a new generation of clinical imaging devices for improved mammography. Beyond the application in conjunction with mammography, the device under development is a precursor to a larger camera. large enough to image the full breast. There is a very large market for such devices and systems which is driven by the need for improved diagnostic tools for breast cancer.